This invention relates to the heat treating of materials, and, more particularly, to techniques for rapidly and selectively heating and cooling in production heat treatment operations.
One of the most important characteristics of many commercial metallic alloys is their capacity for property modification by heat treatment. The basis of heat treatment is the existence of various strength modifying mechanisms such as precipitation hardening and phase transformations such as the formation and modification of martensites. Workpieces made of such alloys may be modified throughout their volumes or only at their surfaces, as needed for particular applications. From a knowledge of the various mechanisms, the properties of such alloys may be modified over wide ranges by the choice of a heat treatment.
Heat treatment generally involves controlled heating and cooling of a workpiece. The result of the heat treatment of a workpiece of a particular composition depends upon a number of parameters such as the temperatures selected, heating and cooling rates, times at temperatures, the use of multiple heating and cooling steps and cycles, and other process parameters.
Some of the commercially most important heat treatments are performed to modify only the surface regions of the workpiece. For example, many steels are specially treated to preferentially harden their surfaces to improve their wear resistance, while maintaining an interior of high toughness and fracture resistance. Where only the surface of the workpiece is to be treated, the depth of penetration of the heated and cooled region into the workpiece and the temperature profiles during the heat treatment are of particular concern. These parameters are often determined at least in part by the shape of the workpiece and the presence of irregularities such as sharp points or recesses at its surface.
Because heat treatment is so important to the most desirable utilization of many materials, many different types of heat treatment apparatus and methods have been developed. One commonly used approach is to heat the workpiece in a furnace heated by gas or electrical elements. After the workpiece has been at the required temperature for the required time, it may be cooled at any of several cooling rates, ranging from very slow furnace cooling to gas cooling to a rapid water quench. This technique processes the entire object at once, but can be very inefficient due to the slow heatup and/or cooling of the workpiece.
In another heat treating approach, a heating beam such as a laser or an electron beam is directed at the surface of the workpiece to heat it rapidly. The surface may instead be induction heated. These techniques are efficient in that they do not heat the workpiece, but they require that the beam or induction coil be moved over the surface of the workpiece. This stepping action can be slow and may result in variability in the heat treatment as a function of location, particularly where there are surface irregularities on the workpiece. Plasma heating has also been tried, but it has not produced rapid heating rates.
The cooling of a workpiece during surface treatments poses similar difficulties. If the workpiece is heated uniformly, rapid cooling is normally achieved by immersing the workpiece in a cooling medium such as water or oil. On the other hand, if the heating source must be stepped over the surface of the workpiece, it is difficult to achieve a uniform, controllable cooling.
There is a need for an improved approach to the heat treating of materials. This need is particularly acute where the heat treatment is a surface heat treatment or where relatively rapid cooling is required, either uniformly over the entire surface of the workpiece or selectively over specific areas of the workpiece. Such treatments are widely utilized in industrial operations. The present invention fulfills this need, and further provides related advantages.